Have you noticed a vibration or rumbling noise when you are driving down the highway? If you have a lot of miles on your vehicle, have you modified the suspension or drive train in any way, you may be experiencing driveline vibration.
Ideally you want the two ends of a double-U-Joint drive shaft within a few degrees of each other for maximum u-joint life and minimum vibration. This is actually the operating angle (under load) and not the angle of the drive shaft to the u-joints themselves (that has its own limit).
Since the rear pinion moves up under acceleration (unless you have anti-wrap control on the axle) ideally you set up the static pinion angle to be 1-2° below the transfer case output flange angle. This way, as the pinion twists up, it comes into a good alignment with the transfer case.
In my case, I stuck on a 1.5" longer shackle and from some simple trig, came up with needing about a 3° shim to compensate for the extra tilt of the shackle. I never measured the angles at the time. Later, I did measure and found even with 3°, I was still 1° above the transfer case and I needed to add another 2-3° to get me passed zero and into the desired range. So I have to conclude that originally my pinion angle was off with the stock shackle as well. Driving experience also confirmed this, I had drive line vibration under load (pinion tips up), but it would go away under coasting conditions (pinion tips down).